1. Field of the Invention
The present invention provides a process for the preparation of glycidyl polyethers of polyphenols using alkali metal salts of polyphenols and 2,3-epoxy-1-haloalkanes as starting materials, and employing a selected group of reaction catalysts.
2. Discussion of the Prior Art
The glycidyl polyethers of polyphenols are more commonly known as "epoxy resins". The resins constitute a class of products characterized by the presence of recurring oxirane units, ##STR3## which lead, after cross linking, to systems with properties that are remarkable in various respects. This has contributed greatly to the development of this type of resins in numerous fields of application.
Among these resins, those conventionally obtained by the reaction of bisphenol A [i.e., 2,2-bis(4-hydroxyphenyl)propane or 4,4'-isopropylidenediphend] and epichlorohydrin (i.e. 1-chloro-2,3-epoxypropane) are of particular interest. It is therefore the object of the present invention to provide a process for the preparation of glycidyl ethers, particularly of bis-phenol A, by the reaction in an anhydrous and aprotic medium of alkali metal salts of polyphenols with 2,3-epoxy-1-haloalkanes. Another object of the invention is to provide a process for the preparation of resins having a viscosity, measured at 25.degree. C., of less than or equal to 100 poises. This type of resin is conventionally prepared by the reaction of bisphenol A with epichlorohydrin in the presence of water and an alkaline agent. Numerous processes are described in the prior art.
For example, U.S. Pat. No. 2,801,227 teaches a process for the preparation of diglycidyl ethers of polyphenols. This process consists of adding an aqueous solution of an alkaline hydroxide to a polyphenol solution in epichlorohydrin at a rate of at least 3 moles of the epichlorohydrin per hydroxyl equivalent of the phenol. The water and part of the epichlorohydrin are distilled from the medium.
In this process, it is essential to control the velocity of the introduction of the alkaline hydroxide solution and the velocity of the distillation so that the reaction medium contains 0.3 to 2% by weight of water. The reaction between the epichlorohydrin and the polyphenol takes place at a temperature of the order of 120.degree. C. In practice, all of the alkaline hydroxide (sodium hydroxide) is only added by the end of 3 to 5 hours. The excess epichlorohydrin is then eliminated by distillation and a solvent is introduced in the reaction medium so as to separate the resin from the salt (sodium chloride) formed.
Another type of process is based on having the alkaline agent play a double role in the envisioned synthesis: a catalytic role to facilitate the condensation of the polyphenol and the epichlorohydrin, and a role as a dehydrochlorination agent to convert the "chlorohydrin" groups into oxirane groups. Such processes are effected in two stages. Thus, French Pat. No. 1,336,444 describes a two stage process whereby a mole of bisphenol A is contacted with at least 10 moles of epichlorohydrin at a temperature of the order of 65.degree. C. in a first stage, while adding progressively, or by increment, an aqueous solution of 50% sodium hydroxide, over a period extending from 2 to 4 hours, the quantity of sodium hydroxide introduced during this stage being less than 16% of the total amount required, this total amount being 2 moles of sodium hydroxide per mole of bisphenol A used.
Distillation then eliminates the water in the form of its azeotrope with epichlorohydrin; in this manner therefore only a portion of the epichlorohydrin is eliminated from the reaction medium.
In the second stage, dehydrochlorination of the remaining epichlorohydrin is effected at about 99.degree. C. by adding the remaining amount of the sodium hydroxide as the dehydrochlorination agent, in the form of pellets. After about an hour, the excess epichlorohydrin is removed by distillation. The resin obtained is then dissolved in a solvent and washed with water. Two phases are obtained, an aqueous phase containing the sodium chloride formed and an organic phase containing the resin. The phases are separated and the organic phase is again treated, for about one hour at approximately 90.degree. C., with sodium hydroxide pellets. The resin is then recovered by various operations of washing, neutralization and drying.
These processes are extremely difficult in operation. Thus, they require relatively long reaction periods, strict control of the different reaction conditions and numerous stages for purifying and/or recovering the resin desired. Furthermore, the loss of epichlorohydrin, which is necessarily used in excess, is significant even if the greatest possible care is taken in the operation of this type of process.
Very recently, certain authors have envisioned another way to obtain this type of resin. Thus, (See MAKROMOL. CHEM. 179, 7, 1661-1671, 1978) it has been suggested that the synthesis of these resins be performed from alkali metal salts of certain diphenols and 2,3-epoxy-1-haloalkanes, in an anhydrous and aprotic medium. However, the reaction is limited by the difficulty encountered in solubilizing the diphenol alkali metal salts. In fact, large amounts of dimethylsulfoxide must be used to render the medium homogeneous; this hinders the development of such a method on an industrial scale.